Rust inhibitive finishes for ferrous metals



United States Patent RUST INHIBITIVE FINISHES FOR FERROUS IVIETALS Byron V. McBride, Irwin, Pa., assiguor to Westinghouse Electric Corporation, East Pittsburgh, Pa., 2 corporation of Pennsylvania No Drawing. Application November 23, 1951, Serial No. 257,982

7 Claims. (Cl. 117-75) This invention relates to rust inhibitive finishes embodying an organic finish applied to ferrous metal surfaces.

At the present time ferrous metal is quite often treated with phosphoric acid compositions in order to prepare the metal surface for a subsequently applied organic finish. For this purpose there are two distinct types of phosphoric acid compositions that are applied to condition the ferrous metal surface. The one type comprises the well known phosphating compositions that produce a protective coating of a phosphate on the ferrous metal. In order to produce such phosphate coatings requires the application of relatively expensive solutions embodying phosphoric acid, a metallic phosphate and an oxidizing agent. The mode of application of these phosphating compositions requires expensive treating apparatus and usually entails several steps of cleaning, drying and sealing with chromic acid in addition to the application of the phosphating composition. The phosphating compositions producing a phosphate coating on a ferrous metal surface have a pH of 5 to 6 or even slightly higher, though slightly acidic at all events. The amount of the applied composition may vary from 25 to 100 milligrams per square foot of surface treated.

The second type of phosphoric acid composition employed in treating ferrous metal surfaces comprises a highly acidic aqueous solution of phosphoric acid alone, or in some cases with additions of a Wetting agent to facilitate wetting or an organic solvent for dissolving oils and greases that may be present on the ferrous metal surface, or both a Wetting agent and an organic solvent. The acidity of these solutions is from a pH of 3 to 0.5 and lower. Usually from 5 to 10 milligrams of the solution per square foot of surface is applied. Such phosphoric acid solutions essentially react with and dissolve the iron oxides on the surface of the ferrous metal and thereby expose clean metal surfaces to which a subsequently applied organic finish will adhere satisfactorily. Ordinarily, there is very little, if any, coating produced on the ferrous metal as a result of treatment with this latter type of composition because of the acidity of the solution.

The present invention is directed to the use of this latter type of strong phosphoric acid solution. When a strong solution of phosphoric acid is applied to a ferrous metal surface to remove iron oxides that may be present thereon, the solution must be removed before an organic finish may be subsequently satisfactorily applied thereto. Any free acid would be detrimental to the protective value of known organic finishes. Ordinarily, after applying the strong phosphoric acid' solution to the ferrous metal, and letting it react with the ferrous metal for a fraction of an hour, the metal is rinsed with water to remove the phosphoric acid, and then dried. Painting of the rinsed ferrous metal must be carried out promptly since the clean, dry metal will rust if permitted to stand exposed to the atmosphere for any length of time.

Since the phosphoric acid in the cleaning compositions functions almost exclusively as a cleaner for the ferrous metal surface and does not produce a protective coating,

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the subsequently applied organic finish does not produce as satisfactory a rust inhibitive finish as produced by the application of the first described phosphating compositions. Thus, where the first mentioned protective phosphate coating inhibits spread of corrosion in case the subsequently applied organic finish is scratched and bare steel is exposed, the second type of phosphoric acid cleaning composition does not produce any significant protective phosphate coating and when the organic finish applied thereto is scratched so that the steel is exposed, rusting will take place rapidly underneath the organic finish, thereby resulting in much less unsatisfactory corrosion inhibiting protection.

The object of this invention is to provide on ferrous metal surfaces rust inhibitive finishes comprising an organic finish and a phosphate reaction product distributed in the layer of organic finish adjacent the ferrous metal surface.

A further object of the invention is to provide a process for coating ferrous metal surfaces with an aqueous solution of phosphoric acid of a pH of 3 and lower and thereafter to apply a primer-surfacer composition containing a pigment reactive with the free phosphoric acid to produce a layer of protective phosphates on and adjacent to the ferrous metal surface.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description.

In accordance with the present invention, a simple process has been devised for treating ferrous metal sur-' faces to provide thereon highly protective rust inhibitive finishes. Briefly, the process comprises (1) applying to the ferrous metal surface a coating of an aqueous cleaning solution of a pH of 3 to 0.5 and lower, said aqueous solution containing phosphoric acid as a major component, the phosphoric acid being reactive with the ferrous surface and the oxides thereon to produce a clean surface to receive a subsequently applied organic finish, the amount of phosphoric acid applied being in substantial excess over the amount required to react with the ferrous surface and the oxides thereon, (2) drying the applied aqueous solution to remove moisture and other volatiles without removing or neutralizing the phosphoric acid, (3) coating the ferrous surfaces with the phosphoric acid present thereon with a primer-surfacer composition comprising resinous film forming ingredients and a large amount by volume of solid pigments reactive with the phosphoric acid to produce phosphates at and adjacent to the ferrous metal surface, (4) drying the applied primer-surfacer composition, and (5) applying a coating of an organic finish over the primer-surfacer coating.

Ferrous metal surfaces to be provided with the rust inhibitive finish of this invention must be cleaned to remove therefrom any gross amounts of dirt, oxides, oils and other matter that may be present that would prevent good adhesion of an organic finish which will be applied thereon. Extremely heavy layers of rust or grease are preferably removed by wire brushing, chipping, solvent treatment or the like as is well known. If only a thin layer of rust or iron oxide such, for example, as a thin blue or red oxide film, is present on the ferrous metal, it may be satisfactorily treated by the process of the present invention without any cleaning pretreatment.

The ferrous metal surface is dipped in or sprayed with, or otherwise coated with, a layer of the aqueous cleaning solution of a pH of 3 to 0.5 or lower. The solution may comprise only water and phosphoric acid. The amount of phosphoric acid should be equal to at least 5% by weight of the solution and may be as much as 50% by weight of the solution though such strong concentra- G3 tions are ordinarily not necessary. However, either a wetting agent or an oil dissolving organic solvent or both may be present.

A cleaning solution that has given good results in practice comprises ethyl alcohol, phosphoric acid and water, with the addition of a wetting agent in the amount of 0.5% of the weight of the solution. The alcohol assists in the dissolving of any oil or grease that may be present on the surface. The Wetting agent is present to permit the solution to wet the ferrous surface rapidly and promptly so that the alcohol may remove the grease and the phosphoric acid will react promptly with any iron oxides and to a slight extent with the ferrous metal surface. A suitable composition for this comprises 100 parts by weight of ethyl alcohol, 100 parts by weight of 85% phosphoric acid and 8G0 parts by weight of Water with 0.5 by weight of a wetting agent.

Another composition comprises 100 parts by weight of 85% phosphoric acid, 560 parts by Weight of Water and 1% of a wetting agent. A further composition comprises 100 parts of 85% phosphoric acid, 100 parts of isopropyl alcohol and 600 parts of water.

Suitable Wetting agents for use in this invention are the esters of sulfosuccinic acid such as the dioctyl ester of sulfosuccinic acid, alkyl and fatty acid polyoxyethylene oxides wherein the alkyl and fatty acid groups contain over four carbon atoms, examples of these are the octadecyl phenyl ethylene oxide condensation product, decyl phenyl ethylene oxide sulfate, and doceyl phenyl ethylene oxide, and sorbitan esters of acids having from 12 to 24 carbon atoms, such, for example, as sorbitan monolaurate and sorbitan tristearate.

The amount of the applied phosphoric acid solution should be at least 800 milligrams per square foot of ferrous metal and preferably between 1000 and 500i) milligrams per square foot. The amount of applied phosphoric acid (HsPOr) should be at least 100 milligrams per square foot of ferrous surface. The acidic aqueous cleaning solution after being applied to the ferrous metal surface will adequately clean the metal surface in a few minutes if the surface oxides are not present in excessive amounts. The cleaning solution may be permitted to react with the ferrous oxides for longer periods of time of as much as one hour.

Thereafter, the members with the cleaning solution are permitted to dry so that the water and alcohol are removed leaving only phosphoric acid present on the metal surface. If a thin layer or coating, of the order of 800 to 1000 milligrams per square foot, of the cleaning com position has been applied to the metal surface, the drying may be eifected by leaving the member exposed to the atmosphere for a brief period of time. More rapid drying is secured if the member is heated with infrared lamps or passed into an oven for a period of time sufiicient to remove the volatile portion of the cleaning solution. In any event, free phosphoric acid will be present on the surface of the ferrous metal. After having been dried, the ferrous metal surface with the phosphoric acid (H3PO4) present in amounts of 100 mililgrams to 500 milligrams per square foot of surface, does not appear wet or damp. However, even if dry in appearance, substantial amounts of free phosphoric acid are present on the surface of the ferrous metal.

Thereafter, the dried, treated ferrous metal surface is coated with a thickness of at least 1 mil of a primer-surfacer composition comprising essentially 100 parts by volume of resinous film-forming ingredients and from 80 to 250 parts by volume of finely divided solid pigments, at least 40% by volume of the solid pigments being composed of at least one compound reactive with phosphoric acid to form a phosphate. The remainder of the solid pigments in the composition may be unreactive or neutral solids, coloring matter and the like. The reactive pigments are preferably selected from at least one of the groups consisting of the oxides, hydroxides, basic silicates,

chromates and carbonates of a metal selected of the group consisting of zinc, lead, calcium, magnesium, manganese, iron, antimony, cadmium and aluminum. Examples of such reactive pigments are zinc oxide, Zinc chromate, litharge, basic lead silicate, red lead, lead carbonate, calcium carbonate, calcium hydroxide, calcium chromate, calcium silicate, magnesium oxide, magnesium chromate, magnesium carbonate, manganese dioxide, manganese carbonate, iron oxide, iron hydroxide, iron carbonate, basic iron silicate, antimony oxide, cadmium carbonate, aluminum hydroxide, leaded zinc oxide, lead oxide, barium chromate and barium carbonate. Furthermore, one or more powdered metals selected from this group of metals may be employed as one of the reactive pigments. Thus, zinc dust, magnesium dust, aluminum flakes, iron powder, lead powder, and cadmium powder may be employed as part of the reactive pigment.

The non-reactive pigments may comprise asbestine (fibrous talc), carbon black, barium sulfate, zinc sulfide, lithopone, titanium dioxide, and calcium sulfate.

The pigments in the primer-surfacer are preferably of a fineness to pass at least through a sieve having 250 meshes to the lineal inch. The pigments for addition to paints are subjected to the usual treatment used in preparing paints, such as ball-milling to an extremely fine subdivision, in order to prevent settling out and to provide for smooth uniform painted coatings.

The volumes of the ingredients in the primer-surfacer are determined on the basis of volume occupied in the dry or cured primer-surfacer coating. For most pigments there are data available indicating the bulking factor so that after a computation of the weight to provide a given volume they may be simply weighed out and added to the solution of resins. Otherwise a graduated container may be filled to a given volume with a resin, solvent added, the pigments are then added and the net increase in volume noted after stirring well.

When the primer-surfacer is applied to the cleaned and dried metal surface, the free phosphoric acid present thereon will react with the reactive pigments therein to produce metal phosphates. The concentration of the metal phosphate is greatest in that portion of the primersurfacer coating near the metal surface and such phosphate reaction products are distributed for some distance within the coating. The amount of the reactive pigment applied in the primer-surfacer coating should be in excess of the amount of the free phosphoric acid so that when these reactive pigments in the primer surfacer have reacted with the phosphoric acid, there will be no free phosphoric acid left on the metal surface.

While a one mil thick coating is adequate, the primer surfacer may be applied in amounts such that a coating of from 5 to 25 mils thick is present, particularly if the primer surfacer is to be sanded or otherwise Worked to produce a smooth surface on the ferrous member being treated. Thus, many castings and rough finished shapes of iron or steel may be coated heavily with the primer surfacer and the primer surfacer is then sanded to produce a smooth surface for a subsequently applied finish coating. At least 5 mils thickness of the primer surfacer is necessary to insure satisfactory sanding.

The applied primer-surfacer coating is dried in order to remove the solvents that are employed in its preparation and to cure the resins. The drying may be accomplished by simple exposure to the atmosphere in case an air-drying resinous composition forms the base of the primer-surfacer. Otherwise, the primer-surfacer coating may be dried by heating lamps or by passing the member with the applied coating into an oven Where it is baked. During the drying operation the resinous component of the primer-surface]: reacts to produce a tough resinous layer carrying a distribution of pigments at and immediately adjacent the ferrous surface. There will be present in the cured coating a substantial proportion of the phosphates of the various metals of the group listed hereinbefore. These metal phosphate pigments have been found to possess a strong rust inhibitive'function. Due to their presence, if the organic finish is cut through by scratching or otherwise, the spread of rusting along the ferrous surface underneath the organic finish will be inhibited thereby.

The following are suitable compositions for the primer surfacer: Example I A composition is prepared containing 50% by volume of pigments and 50% by volume of resin solids. The resin solids comprise 75% of a phenolic resin-tung oil varnish and 25% alkyd resin varnish by weight. The pigments comprise the following parts by volume:

Basic silicate of white lead 30 Titanium dioxide Asbestine Zinc oxide Zinc chromate 10 The resin is dissolved in a solvent and the pigments are added thereto and the whole combined by a bal1- milling operation.

Example II A primer surfacer is prepared by combining the following in which all parts are by volume:

Vehicle:

Alkyd varnish solids 27.4 Phenolic varnish solids 9.4

Pigments:

Basic lead silicate 4.75 Zinc oxide 3.75 Zinc chromate 1.5 Asbestine 12.5 Carbon black 0.13

Xylol was the solvent for the resins. This formulation when ball-milled produces a gray varnish.

Example III The following composition in which all parts are by volume is ball-milled:

Vehicle: Parts Phenolic-tung oil resin 500 Aromatic solvents 500 Pigments:

Calcium carbonate 50 Metallic zinc dust 50 Iron oxide 100 Zinc chromate 50 Magnesium oxide 200 Inorganic dyes may be included in the primer-surfacer composition, particularly such dyes as Prussian blue and ultramarine blue.

After curing of the applied primer surfacer, followed by sanding if necessary, a finish coating of an organic finish is applied to produce a desirable surface appearance. Numerous finishes for this purpose comprising lacquers and enamels, for example, are well known. A suitable composition may comprise a mixture of phenolic resin, tung oil and an alkyd resin with coloring and body pigments. Melamine-alkyd resins or epoxide resins such as set forth in Patents 2,511,913 and 2,456,408 may comprise the base resin in the finishes. The specific pigments and the amounts thereof in the final finish coating are selected to suit requirements. After the final finish is applied, it is cured by baking or air drying and the member is ready for use.

One of the features of the process disclosed herein is that the neutralization or washing or removal of the phosphoric acid cleaning solution is avoided. In fact the presence of excess phosphoric acid is necessary when the primer-surfacer is applied.

The cooperation of the reactive pigments in the primersurfacer with the phosphoric acid produces a highly rust inhibitive protective coating on the ferrous metal as proven by tests. In one test, separate panels of sheet steel were each treated with the same phosphoric acid cleaning solution and both coated with the identical primer surfacer in equal amounts, one set of panels being treated in accordance with the present invention and the other set of panels having the phosphoric acid washed off before applying the primer-surfacer composition. The two sets of panels were subjected to a salt spray test. At the end of 100 hours, the panels produced in accordance with the present invention had no rust spots present whereas the other panels were covered with large rust spots such that they would be considered as having failed completely. Scratches made in the surface of the panels showed that the finish of the present invention prevented in a marked way the spreading of rusting from the scratch beneath the applied paint, with very little rust staining, whereas in the panel in which the phosphoric acid was washed off before the primer-surfacer was applied, rusting had spread considerable distances from the scratch underneath the surface coating and heavy rust stains were present.

Since certain changes in the above invention and different embodiments of the processes and products produced may be made without departing from the scope thereof, it is intended that the above description shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the process of applying a highly protective finish to a ferrous metal surface, the steps comprising (1) applying to the ferrous metal surface a coating of an aqueous cleaning solution of a pH of 3 and lower, said aqueous solution comprising essentially phosphoric acid as a component and being reactive with the ferrous oxides on the surface, the amount of phosphoric acid applied in the coating being in substantial excess over the amount required to react with the ferrous oxides, free phosphoric acid being present in amounts of at least 100 milligrams per square foot of surface, (2) drying the applied coating of aqueous acidic solution without removing the phosphoric acid, (3) coating the dried surfaces with at least one mil thickness of a primer-surfacer composition comprising essentially 100 parts by volume of resinous film forming ingredients and from to 250 parts by volume of finely divided solid pigments, at least 40% by volume of the solid pigments being composed of at least one compound reactive with phosphoric acid selected from the group consisting of the oxides, hydroxides, chromates, basic silicates, and carbonates of a metal selected from the group consisting of zinc, lead, calcium, magnesium, manganese, iron, antimony, cadmium and aluminum, the reactive solid pigments being present in an amount to react with all the phosphoric acid present, (4) drying and curing the applied surfacer-primer whereby a tough adherent resinous coating is produced and (5) applying a coating of an organic finish over the dried surfacerprimer.

2. The process of claim 1 wherein the primer surfacer includes as a part of the reactive pigment at least one finely divided metal reactive with phosphoric acid selected from the group consisting of zinc, lead, magnesium, iron, cadmium, and aluminum.

3. The process of claim 1, wherein the dried primer surfacer is at least 5 mils in thickness and is sanded after curing before the coating of organic finish is applied.

4. The process of claim 1 wherein the aqueous solution includes an oil-dissolving organic solvent.

5. The process of claim 1 wherein the aqueous solution comprises from 5% to 50% by weight of phosphoric acid.

6. A member of ferrous metal having a highly protective surface coating thereon, the surface coating comprising (a) an initial layer of ferric phosphates in direct contact with clean, oxide-free ferrous metal of the member, (b) a primer-surfacer coating of at least one mil in thickness comprising essentially 100 parts by volume of an organic resin and from 80 to 250 parts by weight of inorganic pigments, a substantial part of the pigments being metal phosphates which are most concentrated near the ferrous metal surface and the balance being other compounds, said metal phosphates being the phosphates of at least one metal of the group consisting of zinc, lead, calcium, magnesium, manganese, iron, antimony, cadmium and aluminum, and said other compounds being selected from at least one oxide, hydroxide, chromate, basic silicate and carbonate of the same group of metals, and (c) an organic finish coating superimposed on the primer-surfacer coating.

7. The member of claim 6, wherein there is present in the primer-surfacer coating finely divided metal selected from at least one of the group consisting of zinc, lead, magnesium, iron, cadmium and aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 1,247,668 Gooding Nov. 27, 1917 1,699,274 Bohart .Tan. 15, 1929 1,837,430 Gravell Dec. 22, 1931 2,142,024 Hall Dec. 27, 1938 

1. IN THE PROCESS OF APPLYIING A HIGHLY PROTECTIVE FINISH TO A FERROUS METAL SURFACE, THE STEPS COMPRISING (1) APPLYING TO THE FERROUS METAL SURFACE A COATING OF AN AQUEOUS CLEANING SOLUTION OF A PH OF 3 AND LOWER, SAID AQUEOUS SOLUTION COMPRISING ESSENTIALLY PHOSPHORIC ACID AS A COMPONENT AND BEING REACTIVE WITH THE FERROUS OXIDES ON THE SURFACE, THE AMOUNT OF PHOSPHORIC ACID APPLIED IN THE COATING BEING IN SUBSTANTIAL EXCESS OVER THE AMOUNT REQUIRED TO REACT WITH THE FERROUS OXIDES, FREE PHOSPHORIC ACID BEING PRESENT IN AMOUNTS OF AT LEAST 100 MILLIGRAMS PER SQUARE FOOT OF SURFACE, (2) DRYING THE APPLIED COATING OF AQUEOUS ACIDIC SOLUTION WITHOUT REMOVING THE PHOSPHORIC ACID, (3) COATING THE DRIED SURFACES WITH AT LEAST ONE MIL THICKNESS OF A PRIMER-SURFACER COMPOSITION COMPRISING ESSENTIALLY 100 PARTS BY VOLUME OF RESINOUS FILM FORMING INGREDIENTS AND FROM 50 TO 250 PATS BY VOLUME OF FINELY DIVIDED SOLID PIGMENTS, AT LEAST 40% BY VOLUME OF THE SOLIDS PIGMENTS BEING COMPOSED OF AT LEAST ONE COMPOUND REACTIVE WITH PHOSPHORIC ACID SELECTED FROM THE GROUP CONSISTING OF THE OXIDES, HYDROXIDES, CHROMATES, BASIC SILICATES, AND CARBONATES OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZINC, LEAD, CALCIUM, MAGNESIUM, MANGANESE, IRON, ANTIMONY, CADMIUM AND ALUMINUM, THE REACTIVE SOLID PIGMENTS BEING PRESENT IN AN AMOUNT TO REACT WITH ALL THE PHOSPHORIC ACID PRESENT, (4) DRYING AND CURING THE APPLIED SURFACER-PRIMER WHEREBY A TOUGH ADHERENT RESINOUS COATING IS PRODUCED AND (5) APPLYING A COATING OF AN ORGANIC FINISH OVER THE DRIED SURFACERPRIMER. 